1. Field of the Invention
This invention pertains to an improved Klystron for use as a broadband amplifier. The broadband capability occurs because of the particular distribution of cavities along its length. It is believed that the invention is classified in Class 315, Subclass 5.39.
2. Description of the Related Art
The Proceedings of the IEEE, Vol. 70, No. 11, November 1982, on pages 1308-1310 describes broadband klystron theory. FIG. 10 on page 1309 demonstrates the small signal model of a klystron that may be used to describe the operation of the herein described invention. The bibliography on page 1312 is also of interest.
Broadband microwave tubes are necessary for many uses such as sophisticated communications systems, radars, and countermeasures equipments.
See Chapter 10, "Amplifier Klystrons" of "Klystrons and Microwave Triodes" by Hamilton, Knipp and Kuper, MIT Radiation Laboratory Series, McGraw Hill, 1948.
It is customary to space the cavities of a klystron along the beam at substantially equal intervals. For a given number of cavities tuned to the same frequency, such spacing produces maximum gain because the transconductance of each drift length multiplies those of the other drift lengths. If the transconductances of all drift lengths are equal, for a fixed klystron length their product is maximum.
If the drift lengths are not equal, one drift length is made longer by a certain amount and another drift length shorter by the same amount, and further if the transconductance were proportional to the drift length, the product of these two gains or transconductances would be less than for equal drift lengths. In fact, the transconductance is not exactly proportional to the drift length but varies as the sine of the drift length. The gain of an individual drift length becomes maximum at a certain length, and the product of the transconductances is usually even less for unequal drift lengths than it would be if there were a linear relation between drift length and transconductance.
It is common practice to increase the bandwidth of such klystrons at the expense of gain by stagger-tuning the cavities, or tuning them to different frequencies. Note, however, that because the electrons in a klystron beam are not collected at each cavity but travel from each cavity through all the cavities down stream, the current modulation on the beam of a klystron at a certain cavity is due to all the modulations put onto the beam at all upstream interaction gaps. Thus, due to phase cancellations between all the different current modulation signals, there are zeros in the pole-zero response diagram of a stagger-tuned klystron. The number of zeros is equal to the number of floating resonators.
For a klystron with an electron beam of a given length, as more cavities are introduced, first the number of zeros increases, and second the terms of the gain equation which result from the cascading or multiplication of the transconductances of the individual drift lengths and the impedances of the cavities become smaller in relation to the terms of the gain equation caused by signals which miss interaction at one or more cavities. Consequently, the zeros in the frequency response crowd toward the passband, and there is a limit to how much the bandwidth can be increased by merely stagger tuning the larger number of cavities.
While it is possible to increase the bandwidth by spreading the increased number of cavities out over a longer electron beam, it makes the tube physically larger, and it also increases the problems of magnetic focusing of the electron beam and increases the solenoid electro-magnet power or the energy stored in permanent magnets.
U.S. Pat. No. 3,594,606 which issued July 20, 1971 to Erling L. Lien, assigned to Varian Associates, pertains to the use of second harmonic floating cavities or resonators between the output and input cavities. Applicant adopts the definition of floating resonator or floating cavity recited in this patent. As used herein, a "floating resonator" or "floating cavity" is defined to mean a resonator or cavity which does not have any substantial source of energy external to the microwave tube and which is not coupled to a load utilizing the output of the resonator. However, a circuit may optionally be coupled to the floating resonator solely for effecting some electric characteristic of the floating resonator such as its Q or frequency. Second harmonic cavities are spaced-apart along the electron beam with a fundamental frequency cavity therebetween.
U.S. Pat. No. 3,622,834 which issued Nov. 23, 1971 to Erling L. Lien for a "High-Efficiency velocity Modulation Tube Employing Harmonic Prebunching", is assigned to Varian Associates pertains to a klystron having spaced-apart buncher interaction gaps following an especially long drift tube between the input interaction gap and the first buncher interaction gap
U.S. Pat. No. 3,725,721 which issued Apr. 3, 1973 to Martin E. Levin for an "Apparatus for Loading Cavity resonators of tunable Velocity Modulation Tubes", assigned to Varian Associates pertains to a klystron having a plurality of spaced-apart tunable floating resonator bunching cavities, each having differing resonant frequencies, stagger tuned to broaden the bandwidth of the klystron, according to prior art, the Q of each cavity being determined by the claimed "loading apparatus".
U.S. Pat. No. 4,100,457 issued July 11, 1978 to Christopher J. Edgcombe for "Velocity Modulation Tubes employing Harmonic Bunching", assigned to English Electric Valve Company, pertains to a klystron having spaced-apart buncher cavities with a long drift tube between the last prebuncher interaction gap and the first buncher interaction gap.